It actually really confusing because its purpose is actually serve as a digital currency but this has no real value if not being paired or back up by fiat currency.
But that is how all assets work, asset itself means something that has value for money or useful to someone. If an asset does not have value in fiat then it is impossible to call it asset actually because it has no worth in the market.
Remember that by having bitcoin you should buy it using fiat currency and that determine the value of the bitcoin or any othee cryptocurrency. Now, since the data to access blockchain or where bitcoin is stored in a digital wallet preferrably the hardware wallet so it became a valuable thing and that we can consider this as an asset. Whatever it is cryptocurrency is cryptocurrency and thats it.
Buying bitcoins via fiat only determines the need and that is how the price is determined. Look someone made a coin and today it is worth so much only because of adoption and people understanding the use of bitcoins in the real world. Even hackers demand payment in bitcoins which shows how robust bitcoins mechanism is and it is nearly impossible to hack or steal. These things combine and that's how a decentralized asset got it's market value in fiat.